Binoculars (also known as field glasses), are two identical telescopes mounted side-by-side and aligned to allow the viewer to use both eyes when viewing distant objects. Most of them are sized to be held using both hands. Because of the slightly different viewpoints of each of the viewer's eyes, the binocular gives the users a three-dimensional image.
The Keplerian telescope structure was introduced in 1661 and is based on a positive objective lens, an intermediate image and another positive lens that is the eyepiece. In order to erect the image, two right angle prisms are used. These prisms are mounted between the objective lens and the eyepiece. This structure is the most popular in visual systems. Another popular structure is the Galilean telescope which is more compact. A Galilean telescope consists of a positive objective lens and a negative eyepiece lens with no intermediate image. A Galilean telescope has limited field of view and magnification.
Large field of view enables fast scan of the scene, visualization of more details and visualization of dynamic events simultaneously. Thus, a small field of view is an acute limiting parameter in visual observation optical systems and particularly in binoculars. In general, the two most important features of an optical system are the field of view and the magnification and unfortunately there is a trade of between these two features. In a certain optical system, the product of these two features is constant. For a certain magnification, enlarging the field of view results is a massive and heavy system.
The modern binocular design was presented by Abbe in 1849. Each monocular consists of two major parts, an objective lens and eyepiece. The prisms for image erection, that are mounted between the objective lens and the eyepiece, are the most responsible for the weight of the binocular. This feature results in binoculars that are wide, with objective lenses that are well separated but offset from the eyepieces. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective and wider spacing of the objectives gives a better sensation of depth. Other designs use mirrors in different configurations in order to decrease the bulk and the weight. In some designs, part of the objective lens is located in between the folding mirrors in order to decrease the binocular length.
Binoculars are usually designed for the specific application for which they are intended according to certain optical parameters:    Magnification—The ratio of the focal length of the objective to the focal length of the eyepiece gives the linear magnifying power of binoculars. A larger magnification leads to a smaller field of view.    Objective diameter—The diameter of the objective lens which determines the amount of light that is gathered to form an image.    Field of view—The field of view of a binocular determines the width of the scene that can be seen at 1,000 m, or the angular value of the scene that can be viewed.    Exit pupil—The exit pupil is the image of the aperture stop as imaged by the eyepiece. Actually, the exit pupil is the objective diameter divided by the magnifying power. For the brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye—about 7 mm. However, a larger exit pupil makes alignment of the eye easier and avoids dark vignetting intruding from the edges.    Eye relief—The distance between the exit pupil and the vertex of the eyepiece in the image space is the eye relief. Eye relief determines the distance that the observer must position his/her eye behind the eyepiece in order to see an unvignetted image. The longer the focal length of the eyepiece, the greater is the eye relief. Eye relief is particularly important for eyeglass wearers since the eye of an eyeglass wearer is typically further from the eye piece.
Well-collimated binoculars should produce a single circular, apparently three-dimensional image, with no visible indication that one is actually viewing two distinct images from slightly different viewpoints.
In a regular optical design each of the objective lens and the eyepiece consists of several lenses, some with positive optical power and some with negative optical power. This customary optical design eliminates the curvature of field and corrects the aberrations. The entrance pupil and aperture stop of the optical system are in general identical and are located in the aperture of the objective lens. If the aperture stop is located inside the objective lens or between the objective lens and the intermediate image plane, the eye relief increases but the diameter of the lenses in the front group of the objective lens increases, too. Therefore, the aperture stop is located in an optimal position such that on one hand the eye relief is long enough and the diameter of the exit pupil is suitable, and on the other hand the diameter of the front group of the objective lens is not too large.
There are several optical designs for an eyepiece with a large field of view. In general, an eyepiece with a large field of view consists of two groups of lenses. The negative group is located before the intermediate image and its major role is to correct the distortion and the astigmatism. The positive group is the main part of the eyepiece and is responsible for the positive power of the eyepiece. Both groups correct together the field curvature. In FIG. 1 and FIG. 2, the Nagler and Dilworth eyepieces are illustrated. In the Figures can be seen the large angles of the incoming rays in the eyepiece due to the large field of view. The rays enter to the positive group lenses of the eyepiece with large ascending angles and exit the positive group lenses of the eyepiece with large descending angles. As a result of these large angles the geometrical dimensions of the eyepiece are large, on the order of 60-100 mm.
Therefore, embodiments of the present invention provide an optical design of a binocular with very large field of view without harming other optical parameters, such as exit—pupil Diameter, eye relief, magnification and brightness.
Embodiments of the present invention also provide an optical design of such a binocular which is also compact, light-weight and may be worn by the user.